In continuous ambulatory peritoneal dialysis (“CAPD”), dialysate is instilled into the peritoneal cavity, i.e., the patient's abdominal cavity, via a permanently implanted peritoneal catheter. The dialysate remains in the abdominal cavity for several hours. At the end of this cycle, the dialysate is drained. The exchange of fluid and dissolved substances between the blood in the peritoneal capillaries and the dialysate may result in adequate elimination of substances usually eliminated with the urine.
Known devices for so-called continuous flow peritoneal dialysis (“CFPD”) have a dialyzer that is divided into first and second compartments by a membrane. The first compartment is in communication with the peritoneal catheter via a first sterile circuit, while the second compartment is in communication with a device for delivering dialyzing fluid via a second circuit. The circuits are separated from one another by the membrane of the dialyzer. During CFPD, peritoneal solution from the peritoneal cavity is carried past one side of the membrane, while dialyzing fluid is carried past the other side of the membrane. Exchange of materials between the peritoneal solution and the continuously delivered fresh dialyzing fluid takes place at the membrane. At the same time, toxins on the membrane are removed from the peritoneal solution and, vice-versa, substances resorbed by the patient, such as glucose, are restored.
Owing to the exchange of materials, the concentration gradient of the toxins and the osmotic gradient of the solution in the peritoneal cavity, are maintained relative to the environment, so that a steady secretion of fluid over the peritoneum takes place in the peritoneal cavity. This may result in a practically unlimited increase in fluid in the peritoneal cavity. The aforementioned increase in fluid is also called patient's ultrafiltration.
Therefore, determination of the intraperitoneal volume, i.e., the volume of fluid in the peritoneal cavity, is essential for protection of the patient against “overfilling.” If the intraperitoneal volume is known, the excess fluid can be removed from the first circuit through the membrane of the dialyzer. The known CFPD devices have an ultrafiltration means for this purpose.
The intraperitoneal volume may be determined empirically or by a drain in the patient. EP A 0,149,001 describes a peritoneal dialysis device in which the ultrafiltration rate is controlled as a function of the intraperitoneal volume, so that “overfilling” of the patient is precluded. Ultrafiltration control is based on measurement of dilution. The peritoneal dialysis device comprises a closed circuit, in which dialyzing fluid circulates and to which a substance is added from outside whose secretion and resorption rate is negligible during the entire duration of treatment. The concentration of this substance in the peritoneal solution continuously decreases with increasing volume. During treatment the concentration is measured and compared with the initial concentration at the start of treatment. When a difference between the measured concentration and the initial concentration is detected, the ultrafiltration means withdraws fluid from the peritoneal cavity until the initial concentration in the dialyzing fluid has been reestablished.
A drawback with the device disclosed in EP A 0,149,001 is that an exogenous substance must be added to the dialyzing fluid. Hence, incompatibilities cannot be ruled out. Further secretion or resorption of the substance may result in defective control of the ultrafiltration rate.